Color diffusion transfer photographic film units are generally classified as either separation type or nonseparation type. The term "nonseparation" type film units mean those wherein the separation is not required. Separation type film units are described, for example, in U.S. Pat. No. 2,983,606. Nonseparation type film units are preferable in many applications, such as candid photogaphy, because the transfer images can be seen immediately with pleasure, and the disadvantages of the separation type film units (for example, dirtying of hands caused by processing solutions and litter caused by the separated part of the film unit are avoided. Nonseparation type film units are typically either (1) a unit in which an image receiving layer and a light-sensitive layer are applied to different supports and a processing solution is spread between the image receiving layer and the light-sensitive layer, and (2) a unit in which a processing solution is spread on a light-sensitive sheet composed of an image receiving layer and a light-sensitive layer applied on the same support. The former units are described, for example, in U.S. Pat. Nos. 3,415,645 and 3,415,646 and Japanese Patent Application (OPI) No. 327/72 (corresponding to U.S. Pat. No. 3,619,155) (the term "OPI", as used herein, refers to a "published, unexamined Japanese Patent Application"). The latter units are described, for example, in Japanese Patent Application (OPI) Nos. 60433/81, 157245/82, 157236/82 and 17435/83, U.S. Pat. Nos. 4,374,919 and 4,332,885, and British Pat. No. 1,330,524. In the nonseparation monosheet integrated type photographic film units of the latter type (2), development is started by spreading an alkali processing solution between the light-sensitive sheet and the cover sheet, and development is stopped by reducing the pH of the system by means of a neutralization mechanism in the light-sensitive sheet or the cover sheet, or by release of a development restrainer in the cover sheet. It is of course preferred that the diffusible dyes released as a result of development diffuse into the image receiving layer as quickly as possible to quickly conclude image formation.
On the other hand, after conclusion of development, it is preferred that the image is not changed by for example, a further increase of dye densities caused by additional diffusion of the dyes into the image receiving layer. Such film units are generally designed to provide the most suitable color balance and dye densities at the conclusion of development, and any change of color densities thereafter results in deterioration of image quality. Generally, in monosheet multicolor diffusion transfer film units of type (2), images appear within several seconds to several tens of seconds, and the image formation concludes after several minutes to at most 1 hour. However, in practice the problem of "post-transfer" is observed, i.e., the quality of images deteriorates after 1 hour, because dyes continue to gradually transfer over a period of one to several days.
Film units which solve the above described problem of post-transfer have been described in U.S. Pat. No. 3,730,718. In these film units, a light-shielding layer is provided between an image receiving layer and a light-sensitive element so that the film units after removal from the camera can be developed in the light after exposure. A releasing layer is provided between the light-sensitive element and an element containing the image receiving layer, so that these elements can be separated after development (i.e., as soon as sufficient transfer image densities are observed through a transparent support supporting the above described layers). These film units are essentially separation type film units because they require separation in order to prevent post-transfer, and they necessarily have the disadvantages associated with such separation units discussed above.